1. Field of the Invention
The present invention relates to an optical pick-up having an objective lens for collecting laser light onto an optical disk, and an optical disk device, which collects laser light on an information signal recording layer provided in an optical disk using the optical pick-up to perform the recording and/or reproduction of an information signal.
2. Related Background Art
Hitherto, there has been put to practical use an optical disk device that collects laser light into a fine light spot on an information signal recording layer of an optical disk by using an optical pick-up with an objective lens, to thereby record an information signal, or one that reproduces a recorded information signal from reflected light thereof. Then, in recent, years, in such an optical disk device, it has become possible to raise the numerical aperture (NA) to 0.8, or more, by improvement in the design and manufacturing techniques of an objective lens. By using such an objective lens with a high NA, it is possible to form a finer light spot, to thereby improve the resolution in the recording and/or reproduction, thus, increasing the capacity of the optical disk.
Conventionally, a typical optical disk has an information signal recording layer formed on a substrate of a 0.6 mm to 1.2 mm thickness, which is composed of a transparent resin material. Further, the optical disk device irradiates laser light through the substrate onto the information signal recording layer. However, an objective lens having a higher NA has a shorter focal length, requiring a shorter distance between the objective lens and the information signal recording layer, which makes it difficult to focus laser light through the substrate of a 0.6 mm to 1.2 mm thickness onto the information signal recording layer. Therefore, a proposal has been to make the NA of an objective lens high, and, at the same time, to employ an optical disk having a transparent cover layer thinner than a substrate formed on an information signal recording layer, and to irradiate laser light on an information signal recording layer through the cover layer.
FIG. 8 is a schematic view showing the configuration of such an optical disk device. Here, reference numeral 1 denotes an optical disk, reference numeral 2 denotes an optical pick-up, and reference numeral 3 denotes a spindle motor, which rotates/drives the optical disk 1. The optical pick-up 2 is constructed of a laser light source 6, a collimator lens 7, a beam splitter 8, a collector lens 9, a photodetector 10, an objective lens 20, and an actuator 11, which performs focus control and tracking control of the objective lens 20.
The optical disk 1 is constructed of a substrate 12, an information signal recording layer 13, which is formed on the substrate 12 and consists of a phase changeable material, which is capable of reversibly changing the phase state, and a cover layer 14 having a thickness of about 0.1 mm, which is made of a transparent resin material. On the information signal recording layer 13, a spiral-shaped track or concentric ring-shaped tracks are formed. Incidentally, the information signal recording layer 13 may be made of a magnetooptical recording material or constituted by a metal reflective film having pits (unevenness) formed thereon. Moreover, the optical pick-up 2 is disposed on the cover layer 14 side of the optical disk 1.
FIG. 7 is a schematic view showing the configuration of an actuator 11. The actuator 11 is constructed of a stationary part 15 and a movable part 16. The stationary part 15 is constructed by permanent magnets 17a, 17b, a yoke 18, and a support mount 19. The movable part 16 is a constructed by an objective lens 20, a focus coil 21, a tracking coil 22, and a lens holding member 23 for holding these. Elastic support members 24a, 24b, 24c, 24d each have a linear shape, elasticity, and a high electroconductivity. One end thereof is fixed to the support mount 19, and the other end thereof holds the movable part 16, so as to be freely displaceable in a direction perpendicular to the optical disk 1 surface and in the radial direction of the optical disk 1. In addition, the elastic support members 24a, 24b, 24c, 24d are elastically connected to the focus coil 21 and the tracking coil 22 in the lens holding member 23.
FIGS. 6A and 6B show an objective lens holding structure in the lens holding material 23. FIG. 6A is a plan view (as seen from the optical disk 1 side), and FIG. 6B is a longitudinal cross-sectional view of FIG. 6A. In the lens holding member 23, a through-hole 30, for disposing the objective lens 20, is formed, and there are formed a step-shaped abutting surface 31 on the inner wall surface of the hole 30 and an aperture 32 for restricting the diameter of a light flux at a lower portion of the hole 30. Furthermore, cutout portions 33a, 33b, 33c for adhesion are formed in the upper surface of the lens holding member 23 at the periphery of the hole 30. On the other hand, in the objective lens 20, an edge 34 (a limb provided at the periphery of an optically effective part) is formed. The objective lens 20 is fitted and provided in the hole 30, and is positioned with the lower surface of the edge 34 being abutted on the abutting surface 31. Furthermore, the objective lens 20 is fixed by injecting an adhesive (shown by hatching) into the cutout portions 33a, 33b, 33c for adhesion, which have an open upper end, and by adhering it to the side surface of the objective lens 20.
Further, as shown in FIG. 8, the optical disk device further has an error signal generating circuit 4 and a control circuit 5 for performing focus control and tracking control of the actuator 11. The control circuit 5 supplies a control current to the focus coil 21 and tracking coil 22 through the elastic support members 24a, 24b, 24c, 24d. 
When recording an information signal, first, the optical disk 1 is rotated and driven by the spindle motor 3. With the optical disk being kept rotated, a laser light pulse-modulated in accordance with an information signal generated by the laser light source 6 is made parallel by the collimator lens 7. Further, the parallel light is allowed to pass the beam splitter 8, and is converged by the objective lens 20, to be collected into a fine spot on the information signal recording layer 13 of the optical disk 1 through the cover layer 14.
The information signal recording layer 13 of the optical disk 1 is heated and cooled repeatedly by irradiation of the pulse-modulated laser light. Recording marks, having their phase states changed in an amorphous phase or a crystalline phase, depending on a difference in such a process, are formed as an information signal.
In addition, also when reproducing an information signal, similarly, the optical disk 1 is rotated and driven by the spindle motor 3. In this state, laser light, having a fixed intensity generated by the laser light source 6, is irradiated, so as to be collected into a fine light spot on the information signal recording layer 13 of the optical disk 1 through the cover layer 14. Since the intensity of reflected light from the information signal recording layer 13 at this time will vary depending on the recording marks, an information signal is reproduced thereby.
During such a recording and reproduction operation of an information signal, an optical beam reflected by the optical disk 1 is reflected by the beam splitter 8, and then collected by the collector lens 9, to be detected by the photodetector 10. The photodetector 10 is equipped with light-receiving surfaces divided into a plurality of sections, and the error signal generating circuit 4 generates a focus error signal and a tracking error signal from a detection signal of the respective light-receiving surfaces.
Furthermore, the control circuit 5 supplies a control current, based on this focus error signal and tracking error signal to the focus coil 21 and tracking coil 22, through the elastic support members 24a, 24b, 24c, 24d. The actuator 11 drives the movable part 16 in a direction perpendicular to the surface of the optical disk 1 and in a radial direction, which is at right angles to the recording tracks by an electromagnetic force generated between this control current and magnetic flux generated by the permanent magnets 17a, 17b. 
Thus, even if the optical disk 1 is displaced in the perpendicular direction by surface runout, the light spot is focus-controlled, so as to follow the displacement to be accurately collected on a recording track. In addition, even if a recording track is displaced in the radial direction by decentering, the light spot is tracking-controlled, so as to follow the displacement, to perform scanning.
As described above, the spacing (working distance) between the objective lens 20 and the cover layer 14 surface tends to be reduced in recent years, accompanying the increase in the NA of the objective lens 20. Furthermore, to meet the demand for size reduction of optical disk devices, it is also desired to reduce the size of the objective lens 20, so that it is actually considered to set the working distance thereof to 0.1-0.3 mm.
When the actuator is normally controlled to operate, a given distance is kept between the objective lens 20 and the cover layer 14 surface. However, there are cases when a sudden shock or vibration, or contamination, or a scratch on the optical disk 1, makes it impossible to provide a normal error signal, whereby the actuator 11 may become out of control. At the time of such an abnormal operation, it is unavoidable that the movable part 16 is excessively displaced to come into contact with the optical disk 1.
In addition, in particular, in an optical disk device used for portable equipment, the frequency increases, such that even when not operating, a movable part vibrates by any vibration, shock, or the like, from outside, to come into contact with an optical disk. In such a case, if the objective lens 20 and the optical disk 1 are brought into direct contact with each other, there are cases where either of the members is damaged, so that the recording/reproduction of a normal information signal may become impossible.
Therefore, to solve such a problem, there is proposed, in Japanese Patent Application Laid-Open No. H10-221583, a method of providing a protection member at a periphery of an objective lens as a buffer material for protecting the objective lens. FIG. 9 is a cross-sectional view showing the configuration of the protection member of an optical pick-up described in the above-mentioned official gazette. Reference numeral 35 denotes a protection member provided on an edge 34 of an objective lens 20. The upper surface of the protection member 35, which faces the optical disk 1, is made of a material having a low hardness and slide resistance. Further, the upper surface is at least higher than a vertex of the objective lens 20, and is positioned within the range of the working distance. Hence, the protection member 35 will not contact the optical disk 1 during normal operation, but may come into contact with the optical disk 1 at a time of abnormal operation, whereby, at this time, the objective lens 20 is protected without directly contacting the optical disk 1.
However, the above-mentioned official gazette does not disclose any assembly method of the objective lens holding system. As with the optical pick-up shown in Japanese Patent Application Laid-Open No. H05-189785, an adhesive is injected into cutout portions for an adhesion formed in a lens holding member, and then is adhered to an exposed side surface of an edge of an objective lens. Further, a protection member is similarly fixed using an adhesive. Therefore, the adhesive used for fixing the objective lens and protection member to the lens holding member may run out when applied, even in a slightly excessive amount. As a result, the adhesive may adhere to the disk-side surfaces of the objective lens and the protection member accidentally.
The adhesive adhering to the objective lens or protection member is, generally, not made of a material having a low hardness and slide resistance, unlike the protection member, so that collision between the adhesive adhering to the objective lens or protection member and the disk may damage the disk.